1. Field of the Invention
The present invention relates to a dynamic pressure bearing, specifically a sintered alloy dynamic pressure fluid bearing, and a motor using the same.
2. Description of the Related Art
In a disc reading and writing apparatus, a motor equipped with a dynamic pressure fluid bearing has become in general use as a disc driving motor, because of its high reliability and longevity.
In addition, a radial dynamic pressure fluid bearing made of a sintered alloy, instead of copper alloy that used to be a mainstream material for such a bearing, has now come to frequent use. This is because a sintered material is porous so that it can retain a relatively large amount of lubrication oil therewithin, thereby extending the service life of the bearing. Such a dynamic pressure fluid bearing made of a sintered material has been disclosed in Japanese Patent Application Laid-open Publication No. H11-82479.
In a radial dynamic pressure fluid bearing (referred to as a sleeve, as the case may be, hereinafter) into which a shaft is inserted to be rotatable, a groove, typically a herringbone groove, for generating dynamic pressure is formed in the inner circumferential surface of the sleeve. When the shaft rotates, lubrication oil kept between the shaft and the sleeve flows along the groove toward the apex portion of the groove to increase a pressure of the oil in and around the apex portion, thereby enabling the shaft to be contactlessly supported by the sleeve.
In such a bearing, there are generally provided two herringbone groove portions that are apart with each other along an axis direction in order to support the shaft contactlessly in two positions, thereby preventing the shaft from wobbling as much as possible.
However, when the sleeve is made of a porous material such as a sintered alloy, since the porous material has a countless number of pores, by which the material is referred to as porous, the lubrication oil permeates into the sleeve through the pores and flows therethrough out to somewhere between the grooves where an oil pressure remains lower even when the shaft is rotating.
Therefore, a dynamic pressure of the lubrication oil in the grooves cannot be higher than expected, or is saturated to a certain pressure, at the time of rotation.
Consequently, a shaft supporting capability of the bearing is impaired, which may allow the shaft to yield to a load applied from outside into inclination, resulting in reduced shaft rigidity.
If this takes place, the shaft tends to contact the sleeve and thus be damaged, thereby shortening the service life of the bearing. In addition, a disc reading and writing apparatus employing such a bearing suffers from NRRO (Non Repeatable Run-Out), thereby increasing an occurrence rate of a reading or writing error.
As measures against such a disadvantage of lubrication oil sweeping away through pores in the apex portion of the dynamic pressure groove, the above publication discloses a groove that is similar to the herringbone groove but does not have the apex portion where an oil pressure has to become higher, as illustrated in FIG. 1. According to this type of groove, the lubrication oil is prevented from flowing along a dotted arrow F in FIG. 1 because of a flat portion Mc that eliminates the apex portion in the herringbone groove.
However, in a sleeve 101 having two (a pair of) herringbone groove portions M1, M2 provided apart along an axis direction, since there are no apex portions in the groove portions M1, M2 by the flat portion Mc, the inner diameter d1 of the flat portion Mc has to be smallest in order to generate higher dynamic pressure.
Therefore, the hole having the inner diameter d1 has to be undercut when the bearing having such herringbone grooves is fabricated by press-molding or by plastic forming with a tool, in which a mold or a tool is moved along the inner surface and then removed.
As a result, the mold or the tool tends to have a complicated structure, thereby increasing facility costs.
In addition, when the sleeve is made by plastic forming with a tool, the pair of grooves cannot be made continuously, which is somewhat disadvantageous in that one groove is slightly different in size from the other one of the pair, or their dimensions tend to vary. Namely, it is very difficult to make the grooves of the dynamic pressure bearing with precision, without variation.
On the other hand, the groove M1 can be made in one sleeve and the groove M2 can be made in another sleeve, both of which are then jointed to be in use as a bearing. However, this construction needs a highly precise axial alignment and an increased number of processes, leading to an increased production cost, which makes this construction unfavorable.
The present invention has been made in view of the above disadvantages, and the objective thereof is a low-cost provision of a dynamic pressure bearing and a motor using the same that can generate high dynamic pressure to realize high shaft rigidity and reliability, and the long service life.